Permanent and Induced Magnets

Permanent magnets

A permanent magnet produces its own magnetic field. It has a north pole and a south pole even when no other magnet is nearby, and it keeps its magnetism when an external magnetic field is removed.

When two permanent magnets are brought together, unlike poles attract and like poles repel. Depending on which poles face one another, two permanent magnets can therefore attract or repel.

How an induced magnet forms

Some magnetic materials are not magnets to begin with. If a piece of such a material—such as an iron nail—is placed in the field of a permanent magnet, the field can make the nail magnetic. The nail has become an induced magnet.

The induced poles depend on the nearby permanent magnet. The end of the nail nearest a north pole becomes a south pole; if the permanent magnet is turned around, the end nearest its south pole becomes a north pole. In either case, the nearest poles are unlike, so the nail is attracted towards the permanent magnet.

Induced magnetism is usually temporary. When the material is removed from the permanent magnet's field, it loses all or most of its magnetism quickly. This contrasts with a permanent magnet, which continues to produce its own magnetic field.

Why attraction is not proof of a permanent magnet

A permanent magnet can attract an unmagnetised magnetic material by first inducing magnetism in it. Seeing attraction therefore does not prove that both objects were already magnets.

Repulsion is the decisive test. An induced magnet is attracted to the magnet that induces it, whereas suitably oriented permanent magnets can repel. If an object repels a known permanent magnet when like poles face, the object must itself be a magnet.

The essential difference

Both types can have north and south poles while magnetised, and both can exert magnetic forces. The difference is how their magnetism is maintained:

• a permanent magnet produces its own field and retains its magnetism;
• an induced magnet becomes magnetic because it is in an external magnetic field and normally loses all or most of that magnetism quickly when the field is removed.

Permanent or induced?

• Permanent magnet: produces its own magnetic field, always has north and south poles, and retains its magnetism.
• Induced magnet: becomes magnetic in an external magnetic field and usually loses all or most of its magnetism quickly when removed.

Attraction and repulsion

The pole induced at the end nearest a permanent magnet is the opposite pole, so an induced magnet is attracted.

Attraction is not proof that both objects are magnets. Repulsion is the reliable test for a magnet, because only two like magnetic poles repel.